The attribute or set of attributes
used to uniquely identify each entity instance of an entity
class;
e.g., Student number
for the entity class Student.

Relationship

An association or connection between two or more entities;
e.g., employees work for departments,
students take courses,
libraries order books.

The main aid in finding a relationship is to note
that a relationship is usually a verb.

A relationship between two entities
is either 1-1 (one to one), 1-n (one
to many) or m-n (many to many),
depending on its maximum cardinality.
An example of a 1-1 relationship
might be between Student and Locker,
if each student can have only one locker
and students do not share lockers.
If one of these conditions is broken,
the relationship becomes 1-n;
and, if both are broken,
the relationship becomes m-n.

The membership of an entity class in a relationship
is either mandatory (obligatory),
in which case each entity instance in the class
must be in the relationship,
or optional (non-obligatory).

A relationship may be unary or recursive;
this means that both the entities belong to the same entity
class.
For example, one author cites another;
authors can even cite themselves.

Weak entity

An entity whose existence in the database
is dependent on the existence of another entity.
For example, the entity class Dependent
in an employee data base is usually weak,
because inclusion of information on a dependent
depends on inclusion of information about the employee.

Participation of a weak entity
in at least one relationship is mandatory.